Optical apparatus

ABSTRACT

The present invention provides an optical apparatus including an objective lens system for focusing optical radiation from a scene or object into an intermediate image and has at least one lens element which imposes a substantial degree of negative distortion on the intermediate image. The invention also provides a second lens system for focusing optical radiation from the intermediate image into a final image and an aperture stop for limiting the optical radiation forming the final image. The aperture stop is located between the final image region in which the final image is formed or to be formed and the lens element of the second lens system most distant from the final image region.

FIELD OF THE INVENTION

The present invention relates to optical imaging apparatus, andparticularly, though not exclusively, to apparatus for imaging infra-redradiation.

DESCRIPTION OF PRIOR ART

In forming a viewable image of an object a typical optical imagingapparatus employs an objective lens to focus optical radiation from theobject into an image of thereof for subsequent viewing. Where theoptical radiation is weak or invisible (such as infra-red radiation) itis often necessary to employ intermediate image detecting means todetect the weak image, or to detect an image formed from invisibleradiation.

Many optical imaging methods rely on the use of electronic imagedetectors for this purpose, as does the optical imaging apparatusdisclosed in GB2190761B. In GB2190761B, an optical imaging apparatusemploys an intermediate infra-red detector, in the form of an infra-redstaring array, to detect an invisible infra-red image formed by anobjective lens. The detected image is then produced in a visible form ata display device operatively coupled to the detector. An eyepiece lensis employed by an observer to view the visible image.

In order to improve the resolving power of the detector, GB2190761Bdiscloses the technique of deliberately imposing negative (“barrel”)distortion upon the image formed at the detector, then subsequentlyreversing the barrel distortion with eyepiece optics chosen to impose areciprocal positive (“pin cushion”) distortion upon the image viewed bythe observer. The resolution of the central portion of the image formedat the detector is greater than would be the case were no barreldistortion imposed. The reciprocal action of the eyepiece lenssubstantially removes any such barrel distortion in the viewed image, soas to provide an undistorted image with enhanced central resolution.

However, in many infra-red imaging applications it is unnecessary orundesirable to require that a viewable image be formed by eyepieceoptics. For example, the barrel-distorted image produced by theobjective lens may be detected by suitable thermal imaging devices forproducing electronic image data for subsequent analysis or processingelectronically. In such cases the display device and eyepiece ofGB2190761B are redundant.

Furthermore, a feature of many infra-red imaging devices is the need tocool the detector. Typically, this is done by placing the detectorwithin a dewar cooled to approximately 77K (e.g. by liquid nitrogen, ora cooling engine). In front of the detector and within the dewar isplaced a “cold shield” which shields the detector from stray thermalradiation. Preferably, the cold shield also constitutes the limitingaperture stop of the optical apparatus. The ‘aperture stop’ of anoptical apparatus is that aperture which limits the size of the raybundles passing through the optical apparatus. Alternatively, theaperture stop of the optical apparatus may be located externally of thedewar but as close to the (internal) cold shield as possible.

Thus, it is advantageous that the aperture stop be situated well towardsthe rear of the optical train of the optical apparatus if the detectoris of the cooled type. The objective lenses such as disclosed inGB2190761B employ an aperture stop placed before the objective lens(i.e. in front of the optical train) and are, therefore, not suited tothose electronic detector systems which have a requirement for anaperture stop to be located well towards the rear of the optical train.

Indeed, the location of the aperture stop towards the rear of theoptical train typically leads to unacceptably large diameters for thefront elements of simple existing optical apparatus.

The present invention is concerned with optical imaging apparatus whichprovide enhanced resolution at the centre of the field compared toresolution at the edge, and is particularly concerned such apparatussuitable for imaging utilising a detector coupled to an electronicsignal processing module and where it is a requirement that the aperturestop of the optics be positioned to the rear of the optical train of theapparatus, such as where the detector is an infra red detector of thecooled type within a dewar.

SUMMARY OF THE INVENTION

At its most general, the present invention proposes an optical apparatuswhich produces, before the aperture stop, an intermediate image of aviewed object or scene, the intermediate image being formed with adeliberate negative (“barrel”) distortion, then focussing (relaying)that distorted intermediate image to the detector behind the aperturestop using optics having at least some optical elements located beforethe aperture stop.

Thus, the objective lens elements of the present invention are arrangedto form an image of the viewed object or scene in front of the aperturestop of the apparatus rather than behind it. This intermediate image isthen focussed by the elements of a second optical lens system andreformed at the detector with the aperture stop being placed between thefront of the second optical lens system and the detector. One pictureelement (pixel) at the centre of the field of view (of the relayed anddistorted image) subtends in object space a smaller solid angle than onepixel at the edge of the field, thereby providing enhanced resolution atthe centre. The image formed at the detector may then be processed by anelectronic signal processing module coupled to the detector. Theprocessing may involve the removal of negative distortion from the imageso as to produce a processed image with little or substantially nonegative distortion, but with enhanced central resolution.

In a first of its aspects, the present invention may provide an opticalapparatus comprising;

-   -   an objective lens system for focusing optical radiation from a        scene or object into an intermediate image and having at least        one lens element which imposes a substantial degree of negative        distortion on the intermediate image;    -   a second lens system for focussing optical radiation from the        intermediate image into a final image;    -   an aperture stop for limiting the optical radiation forming the        final image, the aperture stop being located between the final        image region in which the final image is formed or to be formed,        and the lens element of the second lens system most distant from        the final image region.

An image detector (e.g. Infra-red detector) may be employed fordetecting the final image, the aperture stop being located between theimage detector and the lens element of the second lens system mostdistant from the image detector in such a case. The image detector maybe located within a dewar.

It is to be understood that an image “region” (for the final image orthe intermediate image) refers to the region of space across which therespective image extends when formed. Typically such a region is planar,and often referred to as an “image plane”, however, the invention isintended to encompass non-planar images and image detectors employingcorrespondingly non-planar image detecting surfaces.

By employing the above image relaying technique, the present inventionmay provide that all of the optical elements of the objective lenssystem are placed before the aperture stop in the optical train of theapparatus, while at least some of the optical elements of the secondlens system are also placed before the aperture stop. This permitsgreater versatility and ease of manufacture where the detector is aninfra red detector of the cooled type and the aperture stop must beeither within or immediately in front of a dewar. Since none of theobjective optics and not all of (or none of) the elements of the secondoptical system need be placed within the dewar itself (behind theaperture stop), those optical elements may be manufactured as separatemodules from the dewar and its contents.

The aperture stop may be located after all the optical lens elements ofthe second lens system, being located between the image detector and thelens element of the second lens system nearest the image detector. Thisarrangement may provide the advantage that the whole optical train ofthe apparatus of the present invention may be manufactured as a separatemodule(s) from the detector assembly.

As stated above, it is an aim of the invention to provide an opticalapparatus which produces images with high negative distortion, and thenegatively distorting lens elements of the objective are preferablylocated in close proximity to the intermediate image region in which theintermediate image is formed or to be formed. Lens elements situatedclose to an image (or intermediate image) have significant effect upondistortion but little effect upon certain other aberrations such asspherical aberration.

Thus, it is preferable that lens element(s) of the optical train of theapparatus of the present invention which are responsible for negativedistortion of an image are immediately adjacent the image region, or areat least the lens elements of the train closest to the image region, inwhich that image is formed or to be formed (e.g. the intermediate image,or the final image). Consequently, it is preferable that the final lenselement of the objective lens system imparts a substantial negativedistortion on the intermediate image and, more preferably, one or morelens elements of the optical train which precede the final lens alsoimpart a substantial negative distortion on the intermediate image.

Moreover, conventional lens elements with spherical surfaces aregenerally not capable of correcting, or introducing, a high level ofdistortion; it is preferable to use aspheric surfaces to impart negativedistortion on images (intermediate and/or final). Therefore in order toprovide the required high distortion level it is preferable toincorporate one or more aspheric surfaces near to an image region.

Where all of the optical elements of the second lens system are locatedbefore the aperture stop of the optical apparatus of the presentinvention, the optical strength of the negatively distorting lenses istypically required to be very high in order to effect the requireddistortion in the intermediate image. However, it has been found that insuch an arrangement the performance of those lens elements of theobjective lens system responsible for the substantial negativedistortion of the intermediate image tends to be highly sensitive tooptical manufacturing tolerances.

Consequently, in the present invention, at least one optical element ofthe second lens system may impose a substantial degree of negativedistortion on the final image. Thus, in splitting-up and separating theimage (negatively) distorting optical elements between the objectivelens system and the second lens system, the power required of thenegatively distorting lenses has been found to be less than is the casewhere all negatively distorting lenses are located in the objective lenssystem. This has also been found to provide an optical system which isless sensitive to manufacturing tolerances and thereby may provideimproved performance.

The second lens system may comprise only one negatively distorting lenselement or more than one such lens element, with all lens elementsthereof located before the aperture stop. In such a case the distortinglens is preferably the first or the last lens element of the second lenssystem, or where there are two or more such lenses, the first and thelast lens elements of the second lens system are preferably negativelydistorting.

It is preferable to use aspheric surfaces in the negatively distortinglens (or lenses) of the second lens system to impart negative distortionon the final image. Therefore in order to provide the required highdistortion level it is preferable to incorporate one or more asphericsurfaces near to the image region of the final image or the intermediateimage. Preferably, the second lens system has at least two image(negatively) distorting lenses, one located within the optical train ata position relatively near to the image region of the intermediate imageand the other located in the optical train relatively near the imageregion of the final image.

Preferably, the second lens system has a lens element for imparting asubstantial negative distortion on the final image and being the firstlens element following the image region of the intermediate image, and alens element following the aperture stop of the optical apparatus forimparting a substantial negative distortion on the final image and beingthe last lens element preceding the final image region. This ispreferably achieved by providing an image (negatively) distorting lensimmediately following the image region of the intermediate image andplacing another image (negatively) distorting lens after the aperturestop of the apparatus (e.g. within the dewar) and immediately precedingthe final image region. Thus the advantages of separation of the imagedistorting lenses between the objective and second lens systems isprovided, and the advantages of close proximity between negativelydistorting lens and image are also gained since both the intermediateand final images are in close proximity to at least one lens ofsubstantial negative distortion.

Of course, it is to be understood that other optical elements may belocated between the final lens element of the second lens system and thefinal image region. Such other optical elements include the dewar (ordetector) window, and/or a spectral filter, both of which may compriseplano/plano optical components.

Preferably, the final lens element of the objective lens system impartsa substantial negative distortion on the intermediate image. One or morelens elements of the objective lens system which precede the final lensof the objective lens system may impart a substantial negativedistortion on the intermediate image.

Preferably, the detector is an infra-red detector and the aperture stopis located adjacent or within a cooled dewar and serves the function ofa cold shield for the detector. Thus, in such a case, where one or morelenses of the second lens system is located after the aperture stop,those lenses are located within the cooled dewar. The dewar may becooled by use of a coolant such as liquid Nitrogen, or by means of acooling engine. Preferably, the detector is coupled to an imageprocessing module operable to receive image data from the detectorrepresenting a final image detected thereby.

Preferably, the at least some of (preferably all of) the lens elementsof both the objective lens system and the second lens system are chosento be athermal for focus. That is to say, the focal plane position ofeach such lens is substantially constant with temperature over thetypical operating ranges of temperature. One or more of the lenselements of the optical apparatus may possess a diffractive structuresuitable for providing colour correction in the optics.

The optical apparatus may be sold in unassembled form and consequently,in a second of its aspects, the present invention may provide a kit ofparts for an optical apparatus comprising:

-   -   an objective lens system for focusing optical radiation from a        scene or object into an intermediate image and having at least        one lens element which imposes a substantial degree of negative        distortion on the intermediate image;    -   a second lens system for focussing optical radiation form the        intermediate image into a final image;    -   an aperture stop for limiting the optical radiation forming the        final image, the optical apparatus being arranged for locating        the aperture stop between the final image region in which the        final image is to be formed and the lens element of the second        lens system most distant from the final image region in use.

The kit of parts may further comprise an image detector for detectingthe final image, the optical apparatus being arranged for locating theaperture stop between the image detector and the lens element of thesecond lens system most distant from the image detector in use. The kitmay further comprise a dewar for containing the image detector.

Thus, it will be appreciated that the optical apparatus of the presentinvention realises a method of optical imaging. Therefore, in a third ofits aspects, the present invention may provide a method of opticalimaging comprising:

-   -   focusing optical radiation from a scene or object into an        intermediate image with a substantial degree of negative        distortion;    -   focussing optical radiation from the intermediate image into a        final image at a final image region with a lens system;    -   limiting the optical radiation forming the final image with the        aperture stop located between the final image region and the        lens element of the lens system most distant from the final        image region.

A detector having a detection surface (planar or non-planar) may then beemployed (e.g. an Infra-red detector within a dewar) according to thismethod, such that the optical radiation from the intermediate image isfocussed into a final image at an image detection surface with saidoptical lens system.

-   -   The aperture stop would preferably then be located between the        image detection surface and the lens element of the optical lens        system most distant from the image detection surface.

According to this method, it is preferable that a substantial negativedistortion is imposed on the final image while focussing theintermediate image using the lens system. Preferably, a substantialnegative distortion is imparted on an image with lens elements of thelens system located immediately adjacent the image region in which therespective image is formed or to be formed.

Substantial negative distortion may preferably be imparted on the finalimage with a lens element located adjacent the image region of theintermediate image and another lens element located adjacent the imageregion of the final image. More preferably, substantial negativedistortion is imparted on the intermediate image using at least thefinal lens element an objective lens system.

A substantial negative distortion may be imposed on the final imageusing a lens element immediately following the intermediate imageregion, and a lens element following the aperture stop and immediatelypreceding the final image region.

BRIEF DESCRIPTION OF THE FIGURES

Embodiments of the invention will now be described by way of specific,but non-limiting, examples with reference to the accompanying drawingsin which:

FIG. 1 illustrates an optical apparatus in which all optical lenselements are located before the aperture stop of the apparatus;

FIG. 2 illustrates an optical apparatus in which one optical lenselement of the relay lens system is located after the aperture stop;

FIG. 3 illustrates an optical apparatus in which one optical lenselement of the relay lens system is located after the aperture stop, anda lens of the objective system possesses a diffractive surface.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 illustrates a first embodiment of the present invention in whichthe optical apparatus, generally denoted 100, is an infra-red imagingapparatus and operates in the 4.0–5.0 μm infra-red waveband. Theapparatus 100 comprises an objective lens system in the form of anoptical train of lens elements arranged along a common optical axis OA,and consisting of a first (i.e. leftmost in FIG. 1) lens element 101,three successive intermediate lens elements 102, 103 and 104, and aterminal lens element 105. Following this train of objective lenselements, and on the optical axis OA, is a train of relay lens elementsconsisting of a first relay lens element 106, and a terminal relay lenselement 107. A dewar 111 is placed on optical axis OA beyond theterminal lens element 107. The dewar has a window 108 and houses anaperture stop 109 and an electronic infra-red image detector 110. Thedewar assembly is cooled by a suitable means (not shown) which coolsboth the detector 110 and the aperture stop 109, such that it forms acold shield which minimises the ingress of stray thermal radiation tothe cooled infra-red image detector 110.

The optical apparatus has no optical lens elements following theaperture stop (cold shield) 109.

In use, Infra-red radiation from a distant object or scene is incidentfrom the left of FIG. 1 as indicated by rays R. The first two elements,101 and 102, of the objective lens system form a telephoto construction.The first optical surface 1 of the apparatus (at element 101) isspherical and the second surface 2 is aspheric primarily for thecorrection of spherical aberration.

Element 102 carries the third and fourth surfaces of the optical train,surfaces 3 and 4, each of which are spherical. Elements 103 to 105inclusive act together to introduce a large amount of negativedistortion, and surface 8 (the rear surface of element 104) is asphericto assist in achieving this, while surfaces 5 and 6 of element 103, thefront surface 7 of element 104, and both surfaces 9 and 10 of element105 are spherical.

Lens elements 101 to 105 of the objective lens system together form anintermediate image “I” which is distorted negatively and suffers fromadditional aberrations. Elements 106 and 107 form a two-component relaylens system to relay the intermediate image I onto the detector 110,situated within the dewar 111 and behind the cold shield aperture stop109.

Lens element 106 of the relay lens system has an aspheric first surface11, while all other lens surfaces of the relay system are spherical.Lens elements 106 and 107 act in concert with the surfaces 1 to 10 ofthe lens elements 101 to 105 to correct other off-axis opticalaberrations which would otherwise affect the final image “F” formed atthe detector surface 17.

As a result of the interaction of all the surfaces upon the incidentinfra-red radiation, the final image F formed upon the surface of theimage detector 110 is substantially well corrected for all aberrationsexcept negative distortion. The lens elements are chosen such that thisnegative distortion is approximately −50%.

The focal length for axial radiation in this embodiment is 100 mm, whilethe focal length for radiation incident at the edge of the field of viewof the detector 110 is 50 mm (due to the −50% distortion). Thus, thereis a 1:2 ratio in angular subtense (in object space) of a central pixelcompared to an edge pixel.

The refractive materials of the lens elements of the apparatus have beenchosen such that the design is substantially athermal for focus, inother words the focal plane position is substantially constant withtemperature (for temperature variations within the working ranges of theapparatus). The principal athermalisation method is to use materialreferred to in the art as “IG4” for the strongly positive lens elements,but other athermal materials could be used.

The material “IG4” is a proprietary chalcogenide material manufacturedby Vitron Spezialwerkstoffe GmbH, Jena, Germany. This is a materialhaving a refractive index which is inherently relatively stable withtemperature.

It is to be noted, however, that the invention is not confined toathermal systems and it is to be understood that other materials, notproviding an athermal lens design, may be used.

For maximum transmission, it is preferable to use a zinc sulphidematerial known as “CLEARTRAN” for lens element 102. This material is aproprietary product of Rohm and Haas Incorporated.

A particular example of the optical train in accordance with FIG. 1 hasnumerical and material data as follows. The refracting surfaces areindicated from front (leftmost in FIG. 1) to back as surfaces 1 to 17,as has been done in the preceding description. Dimensional units are inmillimeters (but the values are relative and can be scaled accordingly).A positive radius of curvature indicates a centre of curvature to theright of the lens element, and negative curvature to the left. Surface17 is the aperture stop (with aperture ratio F/3.5), and the optimumwavelength is 4.5 microns, the spectral range being about 4.0 microns toabout 5.0 microns, and the focal length is 100 mm.

Design Data:

Surface Radius of Separation number: curvature: Shape: after: Aperture:Material:  1 89.605 spherical 6.250 39.9 IG4  2 −138.61 aspheric 13.13439.1  3 −74.259 spherical 2.273 22.3 Zinc Sulphide  4 74.259 spherical19.820 21.3  5 11.004 spherical 5.876 20.5 IG4  6 10.606 spherical 4.85715.5  7 34.439 spherical 1.705 10.9 Germanium  8 5.574 aspheric 2.5019.2  9 −46.974 spherical 4.861 10.1 Silicon 10 −14.460 spherical 13.04311.8 11 18.239 aspheric 2.273 7.5 Germanium 12 17.244 spherical 5.6446.9 13 −63.155 spherical 2.273 7.8 Silicon 14 −15.447 spherical 2.9018.0 15 infinity flat 1.155 4.7 Silicon 16 infinity flat 0.635 4.4 17infinity flat 12.667 3.75 Image infinity flat 8.6

The curvature of the aspheric surfaces is defined by the equation:

$Z_{Aspheric} = {\frac{{cY}^{2}}{1 + \left( {1 - {\left( {1 + k} \right)c^{2}Y^{2}}} \right)^{\frac{1}{2}}} + {A_{4}Y^{4}} + {A_{6}Y^{6}} + {A_{8}Y^{8}} + \ldots}$where Z_(Aspheric) and ‘Y’ are distances along mutually orthogonal axesin a plane containing, and with their origin at the point where thesurface cuts, the optical axis OA. The quantities c, k, A₄, A₆, and A₈are parameters having the values given below.

SURFACE c k A4 A6 A8 2 −0.00721446 0.000000 1.21110E−06 −9.82437E−11 0.8 0.17940955 −6.499030 6.11132E−04 2.03585E−06 −1.15887E−07 110.05482682 0.000000 −3.56689E−04 1.35698E−06 −1.58352E−07

A disadvantage of the embodiment illustrated in FIG. 1 is that, in orderto provide very high distortion, objective lens elements 103 to 105require strong optical power. This makes them sensitive to manufacturetolerances to the extent that this lens design may be difficult tomanufacture, without undesirable degradation of image quality.

A second embodiment of the present invention, as illustrated in FIG. 2,may provide an optical design which is less sensitive to tolerances andmay provide an even greater level of distortion.

Referring to FIG. 2, there is shown an optical apparatus 200 having athree element objective lens system having a first lens element 201 witha spherical front surface 1′, and a flat rear surface 2′. The secondlens element 202 carries a spherical surface 3′ and an aspheric surface4′, while the terminal lens element 203 of the objective system carriesa spherical surface 5′ and an aspheric surface 6′. This objective lenssystem is arranged to form an intermediate image I after the terminallens element 203, the intermediate image being substantially negatively(“barrel”) distorted principally by the terminal objective lens element.

A three element relay lens system is then provided in the optical trainof the apparatus by relay lens elements 204, 205 and 208. The relay lenssystem begins immediately after the intermediate image plane. Relay lenselement 204 carries an aspheric front surface 7′ and a spherical rearsurface 8′, while both the front and rear surfaces, 9′ and 10′respectively, of element 205 are spherical. The terminal lens element208 of the relay lens system carries an aspheric front surface 14′ and aflat rear surface 15′. Initial and final relay lens elements 204 and 208respectively impart substantial negative distortion on the alreadydistorted intermediate image I they relay.

Element 206 is a dewar window which is not a lens element of the relaylens system. The cold shield aperture stop 207 for the apparatus islocated a short distance after the dewar window 206. It will be cleartherefore that relay lens element 208 is positioned after the aperturestop 207 and directly before the image detector 209 and the final imageplane F thereat.

It is to be noted that the dewar window 206 may be placed between lenselement 208 and image detector 209 in an alternative embodiment in whichthe aperture stop 207 is located outside the dewar 210. Similarly,spectral filters may be placed between lens element 208 and imagedetector 209. Alternatively, the dewar window 206 may be placed betweenthe aperture stop 207 and the final relay lens element 208 in an anotherembodiment where the aperture stop 207 is located outside the dewar 210and the detector window and/or spectral filter(s) may be placed betweenrelay lens element 208 and image detector 209.

Terminal relay lens element 208 may be integrated into the detectormodule (dewar 210) if desired. In this embodiment, refractive materialshave been chosen to make the optical system substantially athermal.Objective lens element 202 carries an aspheric surface 4′ primarily forthe correction of spherical aberration. Objective lens element 203, andrelay lens elements 204 and 208 all carry an aspheric surface asidentified above, and these three lens elements act together to produceabout −60% distortion of the final image F whilst the interaction of allthe lens elements (of the optical apparatus) together providescorrection or substantial correction of other optical aberrations. Theparaxial focal length of the arrangement of FIG. 2 is 100 mm and thefocal length for ray bundles at the edge of the field of view is 40 mm,thereby providing a 1:2.5 ratio between angular subtense (in objectspace) of a central and an edge pixel of the image detector 209.

By incorporating distortion-introducing lens elements close to both theintermediate image I and the final image F, the optical power requiredof these elements is significantly reduced compared to the arrangementillustrated in FIG. 1 where all distortion-introducing lenses arelocated in the objective lens system such that only one element (theterminal one) can be close to an image. By separating thedistortion-introducing lenses across the objective system and the relaysystem, the design of the optical apparatus (such as that illustrated inFIG. 2) becomes much less sensitive to manufacture tolerances, and henceeasier to produce.

A particular example of the optical train in accordance with FIG. 2 hasnumerical and material data as follows. The refracting surfaces areindicated from front (leftmost in FIG. 2) to back as surfaces 1′ to 16′,as has been done in the preceding description of this drawing.Dimensional units are in millimeters (but the values are relative andcan be scaled accordingly). A positive radius of curvature indicates acentre of curvature to the right of the lens element, and negativecurvature to the left. Surface 13′ is the aperture stop (with apertureratio F/3.5), and the optimum wavelength is 4.5 microns, the spectralrange being about 4.0 microns to about 5.0 microns, the semi-field angleis 5.0 degrees, and the focal length is 100 mm.

Design Data:

Surface Radius of Separation number: curvature: Shape: after: Aperture:Material:  1′ 39.889 spherical 5.454 29.8 IG4  2′ infinity flat 4.25328.6  3′ −156.68 spherical 1.909 23.5 Zinc Sulphide  4′ 62.019 aspheric31.884 22.2  5′ 13.985 spherical 1.909 10.1 Germanium  6′ 7.064 aspheric8.539 8.6  7′ −16.094 aspheric 2.136 11.2 Germanium  8′ −10.238spherical 8.895 12.5  9′ 48.635 spherical 1.818 7.6 Silicon 10′ −48.635spherical 2.591 7.3 11′ infinity flat 0.923 3.2 Silicon 12′ infinityFlat 0.508 2.9 13′ infinity Flat 5.793 2.1 14′ 36.360 aspheric 1.909 9.4Germanium 15′ infinity Flat 2.524 9.1 16′ infinity Flat 6.9 (Image)

The curvature of the aspheric surfaces is defined by the equation:

$Z_{Aspheric} = {\frac{{cY}^{2}}{1 + \left( {1 - {\left( {1 + k} \right)c^{2}Y^{2}}} \right)^{\frac{1}{2}}} + {A_{4}Y^{4}} + {A_{6}Y^{6}} + {A_{8}Y^{8}} + \ldots}$where Z_(Aspheric) and ‘Y’ are distances along mutually orthogonal axesin a plane containing, and with their origin at the point where thesurface cuts, the optical axis OA. The quantities c, k, A₄, A₆, and A₈are parameters having the values given below.

SURFACE C K A4 A6 A8  4′ 0.01612411 0.000000 2.01673E−06 3.08170E−092.76137E−11  6′ 0.14156074 0.000000 −8.31031E−04 −7.42175E−07 0.0  7′−0.06213566 3.455575 −1.93918E−04 1.68672E−06 0.0 14′ 0.027502660.000000 8.52164E−04 −1.95061E−05 1.83245E−07

A modification of this embodiment is shown in FIG. 3.

The optical apparatus 300 shown in FIG. 3 has many similarities withapparatus 200 illustrated in, and described above with reference to,FIG. 2. The modification in the apparatus 300 of FIG. 3 arises throughthe use of a different approach to athermalisation and colour correctionas discussed below.

The front lens element 301 of the objective lens array (comprisingelements 301, 302 and 303) carries a provide colour correction andcorrection of spherical aberration. The successive objective lenselement 302 is made of germanium and has strong negative power. Becausegermanium has a high coefficient of variation of refractive index withtemperature, a strongly negative germanium lens element 302 makes asignificant contribution to athermalising the system.

Objective lens element 303, and subsequent relay lens elements 304, 305and 308 perform generally the same function as the corresponding lenselements (203, 204, 205 and 208) illustrated in FIG. 2. Note thatelement 306 is the Dewar window while the terminal objective lenselement 303, the initial relay lens element 304, and the terminal relaylens element 307 all carry aspheric surfaces. The lens elements actingtogether provide −50% distortion, i.e. a 2:1 ratio between axial andedge-of-field focal length, the paraxial focal length is 100 mm.

Using a diffractive surface to provide colour correction permits theoverall length to be reduced compared to the previous embodiment. Theoff-axis ray bundles R come close to satisfying the telecentriccondition at the detector (i.e. the principal rays of the bundles aresubstantially parallel to the optical axis OA). This assists uniformityof illumination.

A particular example of the optical train in accordance with FIG. 3 hasnumerical and material data as follows. The refracting surfaces areindicated from front (leftmost in FIG. 3) to back as surfaces 1″ to 16″,as has been done in the preceding description of this drawing.Dimensional units are in millimeters (but the values are relative andcan be scaled accordingly). A positive radius of curvature indicates acentre of curvature to the right of the lens element, and negativecurvature to the left. Surface 13″ is the aperture stop (with apertureratio F/3.5), and the optimum wavelength is 4.5 microns, the spectralrange being about 4.0 microns to about 5.0 microns, the semi-field angleis 5.0 degrees, and the focal length is 100 mm.

Design Data:

Surface Radius of Separation number: curvature: Shape: after: Aperture:Material:  1″ 35.349 spherical 5.888 29.0 Zinc Sulphide  2″ −360.09aspheric + 6.156 27.8 diffractive  3″ 303.52 spherical 2.386 20.2Germanium  4″ 71.483 spherical 29.238 18.5  5″ 20.661 spherical 2.27310.7 Germanium  6″ 15.393 aspheric 8.482 9.7  7″ −168.65 aspheric 2.71110.8 Germanium  8″ −19.033 spherical 7.730 11.8  9″ 48.757 spherical2.305 9.7 Silicon 10″ −48.757 spherical 3.194 9.3 11″ infinity flat1.154 4.1 Silicon 12″ infinity flat 0.635 3.7 13″ infinity flat 7.2422.7 14″ 51.653 aspheric 2.273 12.1 Germanium 15″ infinity flat 3.15311.8 16″ infinity flat 8.7 (Image)

The curvature of the aspheric surfaces is defined by the equation:

$Z_{Aspheric} = {\frac{{cY}^{2}}{1 + \left( {1 - {\left( {1 + k} \right)c^{2}Y^{2}}} \right)^{\frac{1}{2}}} + {A_{4}Y^{4}} + {A_{6}Y^{6}} + {A_{8}Y^{8}} + \ldots}$

Where the quantities c, k, A₄, A₆, and A₈ are parameters having thevalues given below.

SURFACE c k A4 A6 A8  2″ 0.00277705 0.000000   0.357540E−05−0.172862E−08 0.0  6″ 0.06496640 0.000000  −1.63224E−04  2.98651E−06 0.0 7″ −0.00592953 0.000000  −3.94246E−04  2.13230E−06 0.0 14″ 0.019360000.000000  5.33796E−04  −9.23470E−06 7.00749E−08

And diffractive data concerning the diffractive structure of theaspheric and diffractive surface 2″ of objective lens element 301 isdefined by the equation:

$Z_{Diff} = {\frac{1}{n - 1}\left\{ {\left\lbrack {{H_{2}Y^{2}} + {H_{4}Y^{4}} + {H_{6}Y^{6}} + \ldots}\; \right\rbrack + \left( {\lambda_{0} \times {Int}{\frac{\left\lbrack {{H_{2}Y^{2}} + {H_{4}Y^{4}} + {H_{6}Y^{6}} + \ldots}\; \right\rbrack}{\lambda_{0}}}} \right)} \right\}}$where n is the refractive index of substrate and λ₀ is the designwavelength (0.0045 mm). The quantities n and λ₀ and the quantities H₂,H₄ and H₆ are given below:

SURFACE n λ₀ H₂ H₄ H₆ 2″ 2.24955 0.0045 0.346495E−03 0.0 0.0

The term Z_(Diff) and ‘Y’ are distances along mutually orthogonal axesin a plane containing, and with their origin at the point where thesurface cuts, the optical axis OA. The term Z_(Diff) is an additional Zvalue which arises due to the diffractive structure (i.e. additional tothe aspheric substrate) such that the surface coordinate Z at anoff-axis distance Y on surface 2″ is given by Z_(Aspheric)+Z_(Diff).

The three embodiments described above, with reference to FIG. 1, FIG. 2and FIG. 3, all operate in the 4–5 μm waveband. However, it is toappreciated that the invention could be applied to optical systems foruse in other wavebands, such as the 8–12 μm waveband, providedappropriate refractive materials are used which are transparent in thewaveband of interest. Suitable materials for the 8–12 micron wavebandinclude: germanium; silicon; zinc sulphide; zinc selenide or “KRS-5”(thallium bromo-iodide).

Indeed, it is to be understood that modifications and variations may bemade to various of the parameters employed in the specific examplesprovided above, without departing from the spirit and scope of thepresent invention.

For example, the present embodiments employ a linear optical axis OA,however, the present invention may be applied to an optical systemfolded at a suitable air gap the optical train (such as between elements102 and in 103 of the system illustrated in FIG. 1).

1. An optical apparatus comprising: an objective lens system forfocusing optical radiation from a scene or object into an intermediateimage and having at least one lens element which is designed forimposing a substantial predetermined degree of negative distortion forenhancing resolution at the center of the field on the intermediateimage; a second lens system for focusing optical radiation from theintermediate image into a final image; and an aperture stop for limitingthe optical radiation forming the final image, the aperture stop beinglocated between the final image region in which the final image isformed or to be formed, and the lens element of the second lens systemmost distant from the final image region, wherein at least one opticalelement of the second lens system imposes a substantial degree ofnegative distortion on the final image.
 2. An optical apparatusaccording to claim 1 wherein a lens element of the optical apparatuswhich is responsible for imparting negative distortion on an image islocated next to at least one of the intermediate image region or thefinal image region in which the respective image is formed or to beformed.
 3. An optical apparatus according to claim 2 wherein, the secondlens system comprises at least two lens elements for impartingsubstantial negative distortion on the final image, one lens element ofthe at least two lens elements located next to the image region of theintermediate image and the other lens element located next to the imageregion of the final image.
 4. An optical apparatus according to claim 3wherein, the final lens element of the objective lens system imparts asubstantial negative distortion on the intermediate image.
 5. An opticalapparatus according to claim 2 wherein, the second lens system has alens element for imparting a substantial negative distortion on thefinal image and being the first lens element following the image regionof the intermediate image, and a lens element following the aperturestop of the optical apparatus for imparting a substantial negativedistortion on the final image and being the last lens element precedingthe final image region.
 6. An optical apparatus according to claim 4wherein, one or more lens elements of the objective lens system whichprecede the final lens of the objective lens system impart a substantialnegative distortion on the intermediate image.
 7. An optical apparatusaccording to claim 1 wherein, the aperture stop is located between thefinal image region and the final lens element of the second lens systemnearest to the final image region.
 8. An optical apparatus according toclaim 7 wherein, the final lens element of the objective lens systemimparts a substantial negative distortion on the intermediate image. 9.An optical apparatus according to claim 8 wherein, one or more lenselements of the objective lens system which precede the final lensthereof impart a substantial negative distortion on the intermediateimage.
 10. An optical apparatus according to claim 1 further comprisingan image detector for detecting the final image, the aperture stop beinglocated between the image detector and the lens element of the secondlens system most distant from the image detector.
 11. An opticalapparatus according to claim 10 wherein, the image detector is aninfra-red detector within a dewar and the aperture stop is located nextto or within the dewar and serves the function of a cold shield for theimage detector.
 12. An optical apparatus according to claim 10 whereinthe image detector is coupled to an image processing module operable toreceive image data from the image detector representing a final imagedetected thereby.
 13. An optical apparatus according to claim 1 wherein,one or more of the lens elements of the optical apparatus which areresponsible for imparting substantial negative distortion on eitherintermediate image or the final image incorporate one or more asphericsurfaces.
 14. An optical apparatus according to claim 1 wherein, atleast one of the lens elements of both the objective lens system and therelay lens system are chosen to be athermal for focus.
 15. An opticalapparatus according to claim 1 wherein, one or more of the lens elementsof the optical apparatus possess a diffractive structure suitable forproviding colour correction in the optics of the optical apparatus. 16.A method of optical imaging comprising: focusing optical radiation froma scene or object into an intermediate image with a substantialpredetermined degree of negative distortion for enhancing resolution atthe center of the field; focusing optical radiation from the distortedintermediate image into a negatively distorted final image at a finalimage region with a lens system; limiting the optical radiation formingthe final image with an aperture stop located between the final imageregion and a lens element of said lens system most distant from thefinal image region, wherein a substantial negative distortion is imposedon an image with a lens element being the first lens element followingthe intermediate image region in which the respective image is formed orto be formed.
 17. A method according to claim 16 wherein a substantialnegative distortion is imposed on the final image while focusing lightfrom the intermediate image using the lens system.
 18. A methodaccording to claim 16 wherein a substantial negative distortion isimposed on an image with a lens element being the last lens elementpreceding at least one of the intermediate image region or the finalimage region in which the respective image is formed or to be formed.19. A method according to claim 16 wherein, substantial negativedistortion is imposed on the final image with a lens element of saidlens system located next to the image region of the intermediate imageand another lens element of said lens system located next to the imageregion of the final image.
 20. A method according to claim 19 wherein,the intermediate image is formed by an objective lens system andsubstantial negative distortion is imposed on the intermediate imageusing at least a final lens element of the objective lens system.
 21. Amethod according to claim 20 wherein, a substantial negative distortionis imposed on the final image using a lens element of said lens systemimmediately following the intermediate image region, and a lens elementof said lens system following the aperture stop and immediatelypreceding the final image region.
 22. A method according to claim 16wherein, substantial negative distortion of either or both of theintermediate and final images is provided using lens elements eachcarrying an aspheric surface.
 23. A method according to claim 16wherein, one or more lens elements have a diffractive structure suitablefor providing colour correction.
 24. A method according to claim 17employing an image detector within a dewar wherein said opticalradiation from the intermediate image is focused into a final image atan image detection region of said detector; and, the aperture stop islocated between the image detection region and the lens element of thelens system most distant from the image detection region.